To deepen means to make something deeper or to become deeper. When you dig a hole and then deepen it, you're making it go down further into the ground. A river deepens when erosion carves its channel lower over time.

The word also describes making something stronger, richer, or more intense. A friendship deepens when you and a friend start sharing more honest thoughts and spending more meaningful time together. Your understanding of fractions deepens when you move beyond just memorizing rules and start seeing how they actually work. A mystery deepens when new clues make it more complex and puzzling rather than clearer.

You can deepen your voice by speaking from lower in your chest. A conversation deepens when it moves from small talk about the weather to discussing ideas that really matter to both people. Scientists deepen their knowledge through years of study and experimentation.

The opposite of deepen would be to make something shallower, weaker, or less substantial. When a teacher helps you deepen your thinking about a book, she's pushing you to look beyond the surface and discover layers of meaning you might have missed on your first reading.